During recent years, food scientists have devoted much of their time to developing methods for preparing acceptable meat-like food products, such as beef, pork, poultry, fish, shellfish analogs and extenders and the like, from a wide variety of secondary plant and animal sources. To be acceptable, a food product must be both bland and have a meat-like texture. One such method involves extrusion of a moist proteinaceous material in the form of a plastic mass, under elevated conditions of temperature and pressure, through an orifice into a region of lower pressure to form an expanded porous protein-containing product. This general method is exemplified by the processes of U.S. Pat. Nos. 3,102,031; 3,480,442; 3,488,770 and 3,812,267. Protein extrudate products thus obtained are characterized by their tough, resilient, expanded, open-cellular, rope-like structure made up of interlaced, interconnected pores and channels of varying width and thickness having a bulk density of about 220 to 1500 grams per liter on a weight basis when dried to less than 10 percent moisture. When these protein extrudates are properly hydrated, for instance in boiling water, they will absorb several times their dry weight in liquid and develop a chewy texture somewhat similar to cooked lean meat. Such hydrated products have been used extensively as a partial meat replacement in such food products as chili, stew, meatloaf and the like.
Although these extrusion products represent a significant advance in the art, they do possess several inherent qualities, such as uneven texture and undesirable organoleptic properties, which have limited their general use as a total replacement for real meat proteins, especially where it is desired to simulate the texture of natural lean portions such as beef, pork, poultry, fish or shellfish. For example, it is known in the extrusion art that desirable meat-like texture is dependent upon the degree of protein expansion and the amount of water absorbed by the extrudate during hydration. Protein extrudates which are overly expanded, i.e., those having a bulk density much below about 220 grams per liter on a dry weight basis, are generally regarded as having, when hydrated, a spongy or very porous structure which is too soft to resemble meat. Conversely, a product which has not been sufficiently expanded is too hard and dense to resemble meat. Due to the porous expanded structure of the extrudates, they have the ability to absorb large amounts of water and this creates a problem of moisture control. A protein extrudate which contains too much water is normally too mushy, and a protein extrudate which contains too little water is normally too tough to resemble meat. Unfortunately, it is difficult to control the moisture content of the hydrated extrudates, and the products which result are not totally satisfactory in their meat-like texture.
In addition to the above textural defects, the protein extrudates prepared from crude proteinaceous materials inherently contain undesirable water soluble constituents, such as carbohydrates, salts, flavor and odor components, which are not normally associated with cooked meat products. One means of eliminating these objectionable materials is to extensively purify the proteinaceous starting materials prior to the preparation of the protein extrudates. This prior processing, however, is costly, time consuming and results in loss of valuable proteins.
Some of these flavor problems can be obviated by the water extraction procedures described in U.S. Pat. Nos. 3,142,571 and 3,870,805. These procedures take advantage of the fact that the protein, following a cooking step such as by pressure cooking or by extrusion, is denatured and relatively water insoluble. Thus, the objectionable water soluble materials mentioned above, which are trapped within the extrudates' interstitial spaces or pores, can be partially removed therefrom by aqueous extraction without serious loss of the protein. Removing water soluble materials desirably increases the extrudates' relative protein content to at least about 70 percent on a dry weight basis and allows the extracted protein extrudate to be classified as a textured protein concentrate. However, due to the relatively dense nature of the porous protein extrudates which has been essential to development of meat-like texture, it has not previously been possible to completely remove all undesirable water soluble components, and the resulting products have therefore not been completely bland. In addition, the water content of these hydrated, water extracted protein extrudates is difficult to control without costly and time consuming mechanical water adjusting means and the texture thereof is usually either too tough or too soft to closely resemble real meat.